Overview

Intensive Fixed Sites are the part of the
fixed-site design where all water-column sampling strategies are
employed, as well as all levels of bed-sediment and tissue (see
Bed-Sediment and Tissue
Study), and aquatic ecological assessment sampling. They are
the same as the Basic
Fixed Sitesexcept for the addition of enhanced
sampling frequency and the addition of dissolved-pesticide
analyses for 1 year. The Intensive Indicator
Fixed Sites within each study unit are selected to represent
land-use settings that are most critical for addressing pesticide
contamination issues.

Water-column site selection and sampling
strategy:

The sampling strategy at each Intensive Fixed
Site is the Basic Fixed-Site strategy with the addition of
analyses of dissolved pesticides for all samples and a higher
frequency of interval and extreme-flow sampling during selected
seasonal periods. Intensive sampling and analyses usually are
done for a 1-year period, after which sampling and analyses
return to the same strategy as that of Basic Fixed Sites.
Seasonal periods selected for high-frequency sampling are the
most critical periods for concentrations and transport of
dissolved pesticides and nutrients. The periods are identified
from information on chemical-use patterns, seasonal climatic
conditions, irrigation practices, and existing water-quality
data. During high-frequency sampling, fixed-interval sampling
typically is weekly, or, if the basin is large, biweekly. The
fixed-interval sampling is supplemented with extreme-flow samples
of equal or greater number than those for standard operation of
Basic Fixed Sites (Gilliom and
others, 1995, p. 17).

There are four major components of the
Fixed-Site Reach Assessment conducted by NAWQA at all of the
Intensive (and Basic) Fixed Sites. These components include;
algal, benthic invertebrates, fish-community assessments and
aquatic-habitat evaluations. These data are used to
improve understanding of relations among aquatic biological
communities and the physical, chemical and hydrologic conditions
associated with selected environmental settings (Gilliom and others, 1995, p. 24). In
addition, each aquatic community provides an indication of what
has been occurring in the stream over a different time scale.
Algal communities tend to respond to changes in water quality
relatively quickly (hours to days), aquatic insects somewhat
slower (days to weeks), and fish communities, which are
responding to changes in algal and invertebrate communities as
well as directly to water-quality changes, over a longer period
(weeks to months). An evaluation of aquatic habitat is necessary
to determine if community responses are due to chemical changes
in the water or physical changes in flow, channel morphology, or
other habitat changes.

Algae and aquatic insects are sampled
concurrently in fast flowing water in a known sample area.
Additionally, a qualitative sample of both communities is
collected throughout the stream reach to include a broader range
of habitats and taxa. Fish were sampled with electrofishing gear
appropriate for the stream reach being sampled. The entire stream
reach was fished except in the largest rivers where near shore
habitats were sampled.

Habitat measures were collected along
transects. Both instream and riparian measures are made and
included instantaneous velocity and depth, substrate analysis,
canopy cover, erosion patterns, flood plain and bank composition
and structure, and instream fish-habitat components. Microhabitat
measures were also taken at each algae and aquatic benthic
invertebrate sampling point and included instantaneous velocity,
embeddedness, and substrate particle sizes at each algae and
aquatic benthic invertebrate sampling point.

Additional information regarding sampling
methodology is available in
NAWQA Method and Guideline Protocol publications. A map of
the drainage basins and site locations of the Intensive Fixed
Sites is available for viewing: